1. Field of the Invention
The present invention is directed to a modular ink jet printer head of the type having a plurality of nozzle-containing modules detachably arranged in a holder.
2. Description of the Prior Art
Ink jet printer head composed of edge-shooter ink jet modules can be utilized in many fast printers. These are employed, for example for postage meter machines for franking postal items.
It is known to detachably arrange ink jet printer head modules according to the edge-shooter principle or according to the face-shooter principle in a holder means (First Annual Ink Jet Printing Workshop, Mar. 26-27, 1992, Cambridge, Mass. The modules, however, are spaced from one another by a relatively large distance that is affected by tolerances. This is because the holder means is composed of a plate having oblong openings and two fastening means for each module, whereby the openings lie perpendicularly or obliquely above one another. The time delay of the drive pulses from module to module is therefore high and must be differently set, increased outlay in the control of the drivers. Moreover, a single module cannot be replaced without having to re-program or re-set the time delay for the control.
U.S. Pat. No. 4,703,333 discloses an ink jet printer head constructed of face-shooter modules that are releasably securable in a holder means and are arranged obliquely offset above one another. Such ink jet printer heads having an inclined arrangement of the module relative to the surface of a recording medium produce a more uniform recording even given a fluctuating thickness of the recording medium. The ink jet is no longer perpendicular to, but instead proceeds obliquely relative to, the conveying (transport) direction of the recording medium. A significant disadvantage of face-shooter printers, however, is that face-shooter printers have larger area that resides opposite the recording medium, as a result of which the spacing between the nozzle lines of the modules becomes large and only a few modules can be integrated in an ink jet printer head. This limits the recording density. This disadvantage cannot be completely eliminated either by the oblique arrangement of the modules in the conveying direction of the recording medium or by a laterally offset arrangement. The dimensions of a printer head particularly one operating with under-pressure enter directly into the printing format. The holder means has a common opening for the modules but, has a complicated shape that is correspondingly complicated to manufacture. The manufacture of the printer heads also requires a plurality of manufacturing steps given low tolerances. Guaranteeing the required precision is difficult given such a complicated overall structuring of each and every printer head. The electronic drive of these printer heads having nozzle rows offset relative to one another is likewise complicated.
German OS 32 36 297 provides pre-settable delay networks for the drive of such ink jet printer heads, laterally offset in a field, that are intended to compensate for the spacing of the ink jet printer heads along the conveying direction of the recording medium.
Such ink jet printer heads can, however, only be replaced by a skilled technician who must afterward again carry out the involved mechanical and electrical settings.
When the ink supply is based on capillary action, the ink reservoirs are arranged separated from the printer head and the ink supply pressure must lie within the range of capillary pressure, causing malfunctions to frequently occur in such ink jet printer heads. If the printer head becomes plugged, the entire printer head must be replaced.
PCT Applications WO 91/06432 and WO 91/04861 each disclose an ink jet printer head composed of a single module, which is glued on an aluminum carrier plate and is closely proximate to the ink supply system, or forms a structural unit (printer module) therewith that can be introduced into a holder. The holder has three spherical guide elements that engage in three differently shaped centering openings at one side of the printer module. A plurality of such printer modules would have to be provided for a higher printing format resolution which, however, would then lead, first, to larger dimensions of the overall arrangement and, second, to tolerance problems when introducing the printer modules, so that such printer heads are not suitable for small, lightweight postage meter printers.
U.S. Pat. No. 5,160,945 discloses an edge-shooter thermal ink jet printer head composed of individual modules that contains heating elements for ejecting the ink. Each individual module has a nozzle array and the modules are arranged at identical intervals in the x-direction, secured non-detachably on beam-shaped module carriers which are secured to flanges with pins. The modules are mounted above one another and laterally offset from one another at a spacing in the y-direction. The spacings are relatively large, since the module carriers must already have a relatively large thickness for stability reasons. Identical spacings are extremely difficult to maintain with the module carriers and flanges, particularly when many module carriers are arranged above one another. The outlay for compensating what is only a slight module nozzle density thus becomes too high and the overall structure of the printer head also becomes too large in order to be able to utilize it in postage meter machines. Additionally, the modules cannot be individually replaced.
Another edge-shooter ink jet module that has been proposed is composed of at least three glass pieces, i.e. a middle part having openings and two side parts each having a series of ink chambers. A common row of nozzles is situated at the end face of the first side part. The two rows of ink chambers and the associating nozzles are offset relative to one another, whereby all nozzles in one row lie at the end face of first side part and the ink chambers of the second side part are connected via channels in the middle part to the corresponding nozzles in the first side part, or to the ink supply. An even more highly integrated module can be manufactured according to this principle, which has only a single row of nozzles and forms an edge-shooter ink jet in-line printer head (ESIJIL printer head). A spacer layer composed of the same material as the piezoelectric elements provided for expelling the ink from the ink chambers is arranged on the outside surface of the glass part between the respective sintered blocks of three glass parts, this spacer layer joining the sintered blocks to one another in a non-detachable fashion. If the printer head is damaged during assembly or if the printer head malfunctions during later operation of the printer head, the entire printer head must then again be replaced. However, it is still difficult to achieve a high yield in the manufacture of such printer heads. Heretofore, assembly of edge-shooter ink jet modules to form an ink jet printer head having a high recording density and with low manufacturing costs without a complicated mechanical and electrical adjustment being required has not been successfully achieved, because of the manufacturing tolerances and arising in the printing format.